JP2005253066A - Direct modulation cmos-vco - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control method for controlling a direct modulation voltage controlled oscillator, including preparing modulation variable capacitance and trimming a gain of the modulation variable capacitance. <P>SOLUTION: A variable capacitor modulator includes a differential varactor block, coupling capacitors for connecting nodes of the varactor block to a tank circuit, and a means connected between the respective nodes and ground to trim the gain of the variable capacitor modulator. By employing such a configuration, VOC frequencies are broaden with respect to voltage swing applied on a VCO tank circuit, and power range expansion and modulation index variation are suppressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to the field of wireless communication systems, and more particularly to a variable control oscillator used in a wireless communication system.

  The rapid growth of the market for personal communication systems, wireless medical implant systems and wireless hearing aids has increased the demand for more integrated and more efficient radio frequency (RF) integrated circuits (ICs). These ICs are required to operate with a minimum current consumption at a power supply voltage of less than 2V and sometimes up to 1V at frequencies up to several GHz.

  Recent advances in CMOS technology have greatly improved the transmission frequency of CMOS devices, making CMOS technology one of the promising options for RF integrated circuits enabling a cost effective one-chip solution.

  The transmission unit is one of the most power consuming blocks of the wireless system. In order to save current, it is advantageous to use a direct modulation voltage controlled oscillator (VCO) operating at the transmission frequency as the signal generator of the transmitter. The modulated signal from this VCO is amplified at the output stage and can be applied to the antenna directly or through a filter.

  This scheme is simple and very flexible because the output stage can control the output power over a wide range and the antenna parameters are variable depending on the application.

  A good example of this transmission system is a “1 GHz FM transmitter” operating in an arbitrary 26 MHz band within 100 to 1000 MHz described in Non-Patent Document 1.

  When the antenna is a high Q inductor with a self-resonant frequency that is more than 70% higher than the transmission frequency, the VCO itself can operate as a transmitter, saving current at the output stage.

In this method, the output power range is limited to 12-20 dB depending on antenna parameters and power supply voltage. This is because high power is determined by the power supply voltage and low power is determined by the minimum level of oscillation maintained.
US Pat. No. 6,621,365 NT2800 CHIP-MITTER, www.numatechnologies.com/pdf/NT2800 Andreani “Using CMOS Varactors in RF / VCOs” IEEE J. Solid States Circuits, vol. 35, pp. 905-910, June 20 Chi-Wa Lo “1.5V / 900MHz Monolithic CMOS High-Speed Switching Frequency Synthesizer for Wireless Applications” IEEE J. Solid States Circuits, vol. 37, No. 4, pp. 459-470, April 2002

  A major drawback of the transmission VCO is that the modulation index tends to depend on the transmission power. This is because varactor cells based on MOS capacitance have a relatively narrow voltage control range that is closely related to the MOS transistor threshold. Its normal range is ± 0.5V, while the voltage swing across the tank is up to a few volts. Thus, the VCO frequency is sensitive not only to the control voltage applied to the varactor cell, but also to the voltage swing applied to the VCO tank. This causes either power range limitation or exponential modulation variation.

  The most common direct modulation CMOS VCO configuration is based on the differential method disclosed in NPL 2. In this scheme, two differentially connected varactors or full varactor bridges are used for frequency control (FIG. 1).

  The differential scheme provides the most efficient way to obtain high output and high SNR under power supply voltage limitations. Also, the control voltage is applied to the node where the RF voltage is zero and the control voltage source does not cause further loss in the LC resonance. The varactor block can be connected to the LC tank circuit directly or via a coupling capacitor (Patent Document 1) for reducing the VCO gain (FIG. 2).

  These coupling capacitances also help reduce the RF voltage swing across the varactor and reduce sensitivity between frequency and power.

  Direct VCO modulation can be obtained by applying a modulation voltage to the varactor block in addition to the control voltage that sets the carrier center frequency (FIG. 3).

  The disadvantage of this scheme is that the modulation required for peak-to-peak frequency deviation (at 400 MHz, 0.5-2 MHz) due to the high VCO gain (-10 MHz / V) required for PLL (phase locked loop). The voltage swing must be 0.05-0.2V peak-to-peak. A modulation voltage that is too low makes it difficult to accurately control the modulation index. Also, this low modulation voltage level means a low signal / noise ratio. Also, the VCO gain change according to the RF voltage swing (RF power) affects the modulation index.

  Another way to provide modulation is to add another varactor block to the LC tank in parallel with the varactor block for carrier frequency control. This additional varactor block has another control voltage input and a lower gain (FIG. 4). Lower gain allows higher modulation voltage and higher signal to noise ratio. The modulation index can be controlled not only by the modulation voltage swing but also by the variable capacitance sensitivity of the modulation varactor block, and an appropriate high gain necessary for the PLL loop can be left as it is.

  As noted above, lower gain can be achieved using a varactor block connected to the LC tank via a coupling capacitance. However, the coupling capacitance cannot be used to adjust the gain of the modulation varactor block. This is due to the fairly high voltage on the pin of the coupling capacitance and the CMOS switch for connecting the coupling capacitance segment is smaller due to the smaller gate-source voltage and higher resistance in the RF current path. This is because only the effect is exhibited. Therefore, the resonator Q-factor is destroyed or many parasitic effects are added. A switched capacitor connected in parallel to the modulation varactor block is also not suitable for the same reason.

  A known method disclosed in Non-Patent Document 3 is used to coarsely lower the RF carrier frequency using a switched capacitor connected between the VCO output and ground (FIG. 5).

  This method is not suitable for adjusting the modulation varactor block sensitivity because these switched capacities affect the overall capacity connected to the LC tank, but not dC / dV. Since the frequency depends on the relative tank capacity deviation dC / C (a denominator dependent amount), the modulator gain may appear to be further affected by these capacities, but the PLL is Since the total LC tank capacity is kept constant, the denominator is actually constant.

  A variable modulator capacitance with adjusted gain can be obtained in the manner shown in FIG. 6 in which the upper pins of the two arrays of switched varactors are connected to the LC tank via a coupling capacitance. A modulation voltage to ground is applied to the upper pin via a resistor. The bottom pin of the switched varactor can be connected to ground through an NMOS switch depending on the required modulator gain. The disadvantage of this method is the incomplete utilization of the CV characteristics of the varactor due to the fact that only a positive modulation voltage can be applied to ground. Also, the full power RF voltage and control voltage are applied to the same top pin of the varactor, and the resistance to isolate the modulator voltage source from the RF voltage causes unwanted additional losses and parasitic effects in the LC tank.

  One aspect of the present invention includes a differential varactor block connected to the main LC tank via two, preferably equal coupling capacities, and two, preferably identical, switched trim capacities. A modulation variable capacitor having a gain is provided. Each of the switched trim capacitors is connected between ground and one of the nodes where the coupling capacitor is connected to the varactor block.

  Another aspect of the present invention provides a direct modulation differential VCO having a modulation varactor block connected to the LC tank via two, preferably equal coupling capacitances, and two, preferably the same switched trim capacitances. To do. Each of the switched trim capacitors is connected between ground and one of the nodes where the coupling capacitor is connected to the modulation varactor block.

A VCO according to the principles of the present invention presents the following advantages of the modulation varactor block.
A small modulation varactor block gain set independently of the main VCO gain required for PLL control.
Small impact on the modulation index of low level RF voltage and RF power on the modulation varactor.
• The control voltage is applied to the zero RF voltage node and does not affect the LC tank Q factor.
• Use the entire varactor control voltage range.

  This method also makes it possible to adjust the modulator variable capacitance gain over a very wide range up to a ratio of maximum and minimum gain of 4. This gain change is obtained by a specific volume division effect, and this gain change occurs even when the PLL keeps the total tank capacity value constant. The trim capacitance is connected to minimize Q factor loss. That is, an NMOS switch with minimal resistance to switch the trim capacitor segment can be connected between ground and the bottom pin of the trim capacitor segment so that the bottom pin of the trim capacitor segment that is switched on is always The voltage is almost zero, and the size and parasitic effects of the NMOS switch can be minimized. Using this method, a very small modulation capacitance deviation of 12 fF can be achieved for modulation voltages up to 1V. The additional modulation voltage swing adjustment in this way provides even smaller capacitance changes and very precise control of the modulation index.

  In addition, this method provides a control method for directly controlling a modulation voltage controlled oscillator, comprising preparing a modulation variable capacitor and adjusting a gain of the modulation variable capacitor.

  The invention will now be described by way of example with reference to the accompanying drawings.

    An example of a variable modulator capacitance used in a direct modulation VCO made with a CMOS RF process is described with reference to FIG. The circuit shown in FIG. 7 includes a carrier frequency control varactor block 10 having a varactor 11 coupled to a tank circuit 14 at a node 12. The tank circuit 14 includes a capacitor 22 and an inductance 24 as is known.

  A modulation varactor block 16 having a varactor 21 is coupled to the tank circuit 14 at the node 12 via mutually equal coupling capacitors 20.

  Node 18 is connected to ground through the same array of switched trim capacitors 26. Both the coupling capacitor 20 and the switched capacitor 26 are each composed of a 64 fF MOM fringe basic capacitor cell group. The AMOS varactor cell used in the blocks 10 and 16 usually has the CV characteristics shown in FIG.

  As the control voltage changes from -1.5 V to 0.5 V, the varactor capacitance Cv changes from 0.15 to 0.38 pF, but the useful near-linear region has a narrow range, about -0.9 V to 0. Up to 2V.

  A circuit diagram as an example of all modulator variable capacitors is shown in FIG. Here, a switched trim capacitor 26 is provided at each node. The switched trim capacitor includes a capacitor 30 and an NMOS switch 32.

  The variable capacitance component values are as follows. Each of the coupling capacitors 20 includes 19 parallel-connected 64 fF capacitors, and the total coupling capacitance is Cc = 1.22 pF. Each of the two switched trim capacitors 26 includes 36 cells each having a 64 fF capacitance connected to ground through an NMOSFET (W / L = 3 / 0.35 μm). These 36 switched capacity cells are divided into 7 sets each including 3, 3, 4, 5, 6, 7, and 8 cells and are controlled by a 7-bit temperature code mir <0: 6>. The

  This 7-bit temperature code generates 8 different states with 0, 3, 6, 10, 15, 21, 28, and 36 basic capacity cells turned on (0 to 2.3 pF), respectively. Such trim capacitance grouping provides a non-linear trim capacitance dependency on the code, which ultimately gives a nearly linear modulator variable capacitance gain (delta C versus modulation voltage swing) for the control voltage (FIG. 10). . The larger the trim capacity, the smaller the modulator gain.

  The varactor bridge of FIG. 9 includes four varactor cells 21. Depending on the trim code and the varactor bridge control voltage, the RF voltage applied to the varactor bridge is 0.3 to 0.8 of the total power RF voltage applied to the LC tank 14 due to the capacity division.

  The output differential capacitance value of the modulator variable capacitance for the half bridge voltage is shown in FIG. 10 for three different trim code values. This figure shows that adding the trim capacity increases the total capacity (upper curve from the bottom curve in FIG. 10), but decreases the variable portion of the total capacity.

  A more detailed modulator trimming characteristic is shown in FIG. In FIG. 11, the total output modulator variable capacitance for a mir code is represented by two curves that drop according to the trim code. When mir = 0, all groups of trim capacitors are connected, while when mir = 7, all groups are disconnected. These two curves correspond to the half-bridge modulation voltages of -0.9V and 0.2V, respectively, corresponding to the ends of the useful varactor control voltage range. The peak-to-peak capacity deviation with respect to the trim code when the half-bridge voltage is −0.9 V to 0.2 V is represented by a delta C curve (a curve that rises according to the trim code) in FIG. In this example, the delta C changes substantially linearly from 13 fF to 44 fF according to trim codes 0 to 7. The total modulator capacity is reduced from about 515 fF to 350 fF depending on the trim code. The total capacity change for this trim code is a parasitic effect but is canceled by the PLL loop that keeps the total LC tank capacity constant.

  An example of a simplified diagram of an actual direct modulation VCO circuit having the proposed modulator variable capacitance is shown in FIG. This figure shows the modulation varactor block 16, the carrier frequency control block 10, and the tank circuit 14.

a is a diagram showing a VCO having two conventional varactors, and b is a diagram showing a VCO in which a conventional full varactor bridge is connected in a differential state. a is a diagram showing a varactor block directly connected to the LC tank, and b is a diagram showing a varactor block connected to the LC tank via a coupling capacitor. It is a figure which shows the direct modulation system which uses the same varactor block for carrier frequency control. It is a figure which shows the direct modulation system which uses a separate varactor block for modulation and carrier frequency control. It is a figure which shows the rough adjustment of the LC tank resonance frequency by the grounded switched capacity. It is a figure which shows the modulator gain adjustment by the grounded switched varactor. FIG. 4 is a diagram illustrating modulator varactor capacitance connected in differential to an LC tank according to the principles of the present invention. It is a figure which shows the AMOS varactor cell CV characteristic. It is a circuit diagram of a modulator variable capacitor. It is a figure which shows the differential capacitance value of the modulator variable capacitance with respect to the 1/2 modulation voltage in the case of three different trim code values. It is a figure which shows a modulator variable capacity trimming characteristic. It is a circuit diagram of a voltage controlled oscillator.

Explanation of symbols

10 Carrier frequency control varactor block 11, 21 Varactor 12, 18 Node 14 Tank circuit 16 Modulation varactor block (modulation variable capacity / variable capacity modulator)
20 Coupling capacity 22 Capacity 24 Inductance 26 Switched trim capacity

Claims (12)

A variable capacitance modulator used in a voltage controlled oscillator,
A differential varactor block;
A coupling capacitor connecting the node of the varactor block to the tank circuit;
Means for adjusting the gain of the variable capacitance modulator connected between the node and ground.   The variable capacitance modulator of claim 1, wherein said means comprises a plurality of switched trim capacitors.   The variable capacitance modulator according to claim 2, wherein the switched trim capacitors are the same.   4. The variable capacitance modulator according to claim 1, wherein the switched trim capacitor includes an NMOS switch. A differential varactor block;
A plurality of coupling capacitors connecting a pair of nodes of the varactor block to a tank circuit;
A direct modulation voltage controlled oscillator comprising a plurality of switched trim capacitors connecting the pair of nodes to ground.   The direct modulation voltage controlled oscillator according to claim 5, wherein the plurality of coupling capacitors are substantially equal to each other.   7. The direct modulation voltage controlled oscillator according to claim 6, wherein the plurality of switched trim capacitors are the same as each other.   The direct modulation voltage controlled oscillator according to claim 7, wherein the plurality of switched trim capacitors constitute an array.   9. The direct modulation voltage controlled oscillator according to claim 5, wherein the tank circuit is connected to the plurality of coupling capacitors.   10. The direct modulation voltage controlled oscillator according to claim 5, wherein the coupling capacitor and the switched trim capacitor are configured by a MOM fringe basic capacitor cell group. A method for controlling a direct modulation voltage controlled oscillator, comprising:
Preparing a modulation variable capacitor;
Adjusting the gain of the modulation variable capacitor. The method of claim 11, wherein the gain is adjusted using a switched trim capacitance to minimize Q factor loss.

Images